The invention relates to a method for priming a substrate by contacting the substrate with a primer fed from a primer source and depositing the primer on the substrate. The invention also relates to a process for the coating of a substrate by contacting the substrate with a primer fed from a primer source, depositing the primer on the substrate, and coating the primed substrate with a coating substance.
There are several methods of improving the adhesion between a substrate and its coating. These methods can be surface treatment, mechanical roughening, removing weak boundary layers, minimising stresses, using adhesion promoters, using suitable acid-base interactions, as well as providing favourable thermodynamics and using wetting. Typical treatment techniques include the use of chemicals such as primers and solvents, the use of heat and flame, mechanical methods, plasma, corona treatment and radiation. Each technique can have several effects that improve adhesion.
An important method of improving the adhesion between a substrate and its coating is priming. Priming means the treatment of a substrate with a primer. A primer means a prefinishing coat applied to surfaces that are to be painted or otherwise finished. See McGraw-Hill Dictionary of Scientific and Technical Terms, 6th Ed., p. 1668 and 1669.
Typical primers are adhesive organic substances which are soluble in water and/or an organic solvent and are used for treating the substrate surface in order to improve its adhesion or bonding to the coating. In the following table, typical primers and their adhesion and performance characteristics are given.
TABLE 1Properties of typical primersAdhesion CharacteristicsPerformance CharacteristicPlasticHeatMoistureChemicalType of primerPaperMetalfilmResistanceResistanceResistanceShellacPoorExcellentPoorPoorPoorPoorOrganic TitanateGoodGoodGoodFairFairFairPolyurethaneVery goodExcellentExcellentExcellentExcellentExcellentPolyethyleneimineVery goodGoodExcellentExcellentPoorPoorEthylene AcrylicExcellentExcellentFairFairExcellentGoodAcidPolyvinylideneExcellentFairExcellentGoodVery goodFairChloride
Traditional priming takes place by conventional solution application techniques. The application of a primer promotes adhesion between the substrate and the coating by increasing the free energy (wettability) of the surfaces, inducing chemical reaction between them, and removing bond weakening impurities from them.
However, traditional priming has the drawback that it is difficult to achieve the correct coating weight suitable for the specific primer to be used. Uniform deposition is important for all primers. This is especially the case with uneven surfaces, the less available sites of which are poorly reached by conventional priming techniques.